The present invention relates to a radiological imaging apparatus, and more particularly to a radiological imaging apparatus preferably applicable to X-ray CT, Positron Emission Computed Tomography (hereinafter, referred to as PET), Single Photon Emission Computed Tomography (hereinafter, referred to as SPECT) or the like.
Radiological imaging technology using radiation can examine conformation of a subject non-invasively. In particular, examples of radiological imaging of a human body as a subject include X-ray CT, PET and SPECT. In any of the technologies, by measuring a physical amount of integrated value (flying direction) of radiation radiated from the human body and projecting back the integrated value, the physical amount of each voxel in the human body is calculated and imaged. Enormous data needs to be processed for imaging. Rapid progress of computer technology in recent years enables to provide tomographic images of the human body at high speed with high accuracy.
In X-ray CT, an examinee is irradiated with X-ray from an X-ray source and an intensity of X-ray passing the conformation of the examinee is measured and from a rate of X-ray passing the conformation, mode information on sections of the examinee is imaged, that is, tomographic image of the subject is obtained. More specifically, the intensity of X-ray passing the conformation of the examinee is measured by a radiation detector arranged on a side opposed to the X-ray source relative to the examinee and by using the measured X-ray intensity, a linear attenuation coefficient between the X-ray source and the radiation detector is calculated. Since transmitted X-ray is measured by turning the X-ray source and the radiation detector around the examinee, a distribution of the linear attenuation coefficient in the conformation is calculated. The linear attenuation coefficient of each voxel is calculated by using the Filtered Back Projection Method described in the IEEE Transaction on Nuclear Science NS volume 21, page 21 and the value is converted into a CT value. The radiation source often used in the X-ray CT is around 80 keV.
The PET is a method of administering to the examinee, radio pharmaceuticals (hereinafter, PET pharmaceuticals) containing matters having a property of concentrating on positron emitters (15O, 12N, 11C, 18F, etc.) and specific cells in the body and examining locations in the body where more PET pharmaceuticals are consumed. An example of radio pharmaceutical is (2-[F-18] fluoro-2-deoxy-D-flucose, 18FDG). 18FDG is highly concentrated on tumor tissue by carbohydrate metabolism and therefore is used to specify location of tumor. A positron emitted from a positron emitter in the PET pharmaceutical concentrated on a specific location, couples with an electron of a neighboring cell to disappear and irradiates a pair of γ-rays having an energy of 511 keV. These γ-rays are irradiated in directions substantially opposed to each other (180°±0.6°). When the pair of γ-rays is detected by a radiation detector, it is found at which pair of the radiation detectors the positron is emitted. By detecting a number of the pairs of γ-rays a location where the PET pharmaceuticals are more consumed is found. For example, 18FDG concentrates on a cancer cell with enhanced carbohydrate metabolism as described above and therefore, cancer focuses can be discovered by PET. Further, the obtained data is converted into a radiation generating density of each voxel by the above-described filtered back projection method to thereby contribute to imaging of a location of generating the γ-ray (a location where radiation radiateters are concentrated on, that is, a location of the cancer cell). 15O, 12N, 11C and 18F used for the PET are radioisotopes having a short half life of 2 to 110 minutes.
According to the examination by PET, data obtained by PET examination is corrected by using the data of a transmission image picked up using a γ-ray source. The transmission image is provided by a method of measuring an attenuation rate of γ-ray in the body by irradiating the γ-ray by using, for example, cesium (γ-ray source) and measuring an intensity of γ-ray passing the body of the examinee. A PET image having high accuracy can be provided by estimating the γ-ray attenuation rate in the body by using the obtained γ-ray attenuation rate and correcting data obtained from the PET.
SPECT administers to the subject, radio pharmaceuticals (hereinafter, referred to as SPECT pharmaceuticals) including single photon radiateters to examine γ-ray radiated from the radiateters by a radiation detector. The energy of γ-ray radiated from single photon radiateters often used in examining by SPECT, is around several 100 keV. In the case of the SPECT, single γ-rays are radiated and therefore, an angle incident on the radiation detector cannot be provided. Hence, angle information is obtained by detecting only γ-ray incident from a specific angle by using a collimator. The SPECT is an examination method which administers to the examinee, SPECT pharmaceuticals containing matters having a property of concentrating on a specific tumor or molecules and single photon radioteters (99Tc, 67Ga, 201Tl, etc.), detects the γ-rays generated by the SPECT pharmaceutical and specifies a location where the SPECT pharmaceutical is more consumed (for example, a location where a cancer cell is present). The SPECT also converts the obtained data into data of each voxel by the method of filtered back projection or the like. Further, the SPECT also picks up the transmission image frequently. 99Tc, 67Ga, 201Tl used for the SPECT are provided with a half life longer than that of radioisotopes used in the PET, for example, 6 hours to 3 days.
As described above, the PET and SPECT can sample with good contrast a location where the radio pharmaceutical is integrated since a functioned image is obtained by using metabolism in the body; however, there poses a problem that a positional relationship with surrounding organs cannot be grasped. Hence, in recent years, attention is attracted to a technology of carrying out a diagnosis to a higher degree by synthesizing a mode image which is a tomographic image obtained by X-ray CT and a functioned image which is a tomographic image obtained by PET or SPECT. An example of this technology is described in Japanese Patent Laid-open No. 7-20245.
According to a radiological imaging apparatus described in Japanese Patent Laid-open No. 7-20245, an X-ray CT image pickup apparatus and a PET image pickup apparatus are installed in series and the examinee is examined by using the two image pickup apparatus by moving a bed on which the examinee is laid in the horizontal direction. In this case, a time interval of carrying out the two examinations is short, the examinee hardly moves on the bed and therefore, a corresponding relationship between PET data and X-ray CT data which are imaging data obtained by the two image pickup apparatus is found. By using the corresponding relationship, PET data and X-ray CT data are synthesized and the focus location of the examinee is specified.
Japanese Patent Laid-open No. 9-5441 describes a radiological imaging apparatus serving also as a bed and arranged with an X-ray CT image pickup apparatus and a SPECT image pickup apparatus in series. X-ray CT data and SPECT data which are image pickup data obtained from their respective image pickup apparatus are synthesized and the focus location of the examinee is specified.
Although the radiological imaging apparatus described in the publications are apparently clear in the positional relationship between the two image pickup data, there is a possibility that the examinee constituting the subject is moved between the two image pickup apparatus. Resolution of the PET image pickup apparatus in recent years is about 5 mm and resolution of the X-ray CT image pickup apparatus is about 0.5 mm, smaller than the above-described resolution substantially by one digit. Therefore, when the examinee is moved between the two image pickup apparatus or an angle of the examinee is changed, the corresponding relationship of the respective image pickup data obtained by the two image pickup apparatus become unclear. As a result, after reconstituting images of the respective image pickup data, it is necessary to sample characteristic areas commonly present in the respective images and calculate the positional relationship between the respective images from the positional relationship of the characteristic areas to thereby position the images. Further, the radiological imaging apparatus are provided with two of image pickup apparatus respectively having radiation detectors and the like and therefore, the constitution of the apparatus is complicated.